This invention relates to optical fiber communication systems, and more particularly to an optical fiber-communication system that employs remote pumping of the optical amplifiers.
Commercial lightwave systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the loss associated with the optical fiber (along with chromatic dispersion and optical nonlinearity). To transmit optical signals over long distances, the lightwave systems may include a number of repeaters periodically located along the fiber route from the transmitting terminal to the receiving terminal. Each repeater boosts the weak signal to compensate for the transmission losses which occurred enroute from the last repeater. Prior to the widespread availability of efficient optical amplifiers, many systems converted the optical signals into electrical signals for amplification by conventional electrical amplifiers. The amplified electrical signals were then reconverted to the optical domain, for further distribution along the optical communication path. The advent of reliable and low cost optical amplifiers has obviated the need to convert signals into the electrical domain for amplification. However, optical amplifiers, such as rare earth doped optical fiber amplifiers, require a source of pump energy. In a rare earth doped optical fiber amplifier, for example, a pump laser is coupled to the doped fiber for exciting the rare earth element within the amplifier. At the same time, a communication signal is passed through the doped fiber. The pumped fiber exhibits gain at the wavelength of the communication signal, providing the desired amplification.
One disadvantage in using pumped optical amplifiers is that electrical energy must be provided at the amplifier location in order to operate the pump laser. This requirement can be particularly burdensome if the transmission system, and hence the optical amplifiers, is located undersea. In undersea transmission systems, power must be supplied to remote undersea locations. Furthermore, repair of active components such as pump lasers can be prohibitively time-consuming and expensive. U.S. application Ser. No. [Kerfoot 4] overcomes this disadvantage by eliminating the need to have all elements of the individual optical amplifiers co-located. In this reference, an optical fiber pump path, which is distinct from the optical transmission path, optically couples the pump source to the plurality of doped optical fibers. Accordingly, the active components, i.e., the optical pump source, may be remotely located from the passive components, i.e., the doped optical fibers and couplers. It would be desirable, however, to eliminate the need for a dedicated pump path.
In accordance with the present invention, a lightwave communication system is provided that includes first and second terminals remotely located with respect to one another. First and second optical transmission paths couple the first terminal to the second terminal for bidirectionally transmitting optical information therebetween. First and second doped optical fibers are respectively disposed in the first and second optical transmission paths. Optical pump energy is supplied by first and second optical pump sources. The first optical pump source generates Raman gain in the first transmission path and the second optical pump source generates Raman gain in the second transmission path. A first optical coupler is provided for optically coupling pump energy from the first transmission path to the second doped optical fiber and a second optical coupler is provided for optically coupling pump energy from the second transmission path to the first doped optical fiber.